For crust, in a large mixing bowl stir together flour, the 2 tablespoons cocoa, and salt. Using a pastry blender, cut in shortening and 1/3 cup butter until pieces are pea-size. Sprinkle 1 tablespoon of water over the mixture; gently toss with a fork. Push moistened dough to the side of the bowl. Repeat, using 1 tablespoon of water at a time, until all the dough is moistened.
Preheat oven to 450 degrees F. Divide dough in half. Form each half into a ball. Cover and refrigerate one ball. On a lightly floured surface, flatten remaining ball. Roll from center to edges into a 12-inch circle. Transfer to a 9-inch pie plate; avoid stretching. Gently press into plate. Trim edges. Line with double-layer of foil.
Bake pastry for 8 minutes. Remove foil; bake 5 minutes more or until firm and dry. Sprinkle with the semisweet chocolate chips. Cool completely on a wire rack. Reduce oven temperature to 325 degrees F.
Slowly stir 1 cup of the hot mixture into the beaten egg yolks. Pour egg yolk mixture into hot filling in pan. Bring just to boiling; reduce heat. Cook and stir for 2 minutes. Remove from heat. Stir in butter and vanilla. Let cool 10 minutes.
Meanwhile, on a lightly floured surface, flatten chilled dough. Roll from center to edges to form a 12-inch circle. Pour warm filling mixture into baked pastry shell. Place pastry top over the filling mixture, pressing into pastry edge to seal. Trim dough to edge of pie plate.